Rigged low poly charector deforms when moving
You've missed adding two of the upper arm vertices to the arm vertex group so they stay in their original position while the rest of the arm moves. In Edit mode, select the arm.l vertex group by hilighting it in the R/H pane and clicking 'Select'. Then Shift-click on each of the vertices I've indicated on the model and click the 'Assign' button in the R/H pane it should work ok. You seem to have done the same with most of the head vertices. That shows an even worse problem!